The present invention is directed to thermoforming and, more particularly, to an apparatus for thermoforming polyesters into articles such as ovenable containers.
Continuous vacuum-forming devices for making containers from thermoplastic sheets typically utilize a continuous sheet of molten plastic which is extruded and vacuum-formed on a continuous belt or a rotating drum having a plurality of mold cavities. Many of these devices utilize residual heat from the extrusion process, thus avoiding the need to reheat the plastic sheet prior to thermoforming. It generally is considered desirable that heat-set articles such as ovenable containers have relatively uniform thermal crystallinity throughout the article to provide adequate dimensional stability and impact resistance. For example, Demerest U.S. Pat. No. 5,614,228 describes a continuous rotary thermoforming apparatus in which a sheet of molten polyethylene terephthalate is extruded and vacuum-formed on a rotating drum having a plurality of mold cavities around its circumference. A hot oil circuit and electric heating elements are provided to impart different amounts of heat to different locations of the sheet during thermoforming. According to Demerest, additional heat is applied to portions of the articles which have a greater wall thickness to produce more uniform crystallinity throughout the article. The sheet is required to be tensioned and oriented during article forming to prevent the sheet from warping or otherwise distorting during cooling. Orienting the sheet also is said to result in articles having high impact resistance.
Several drawbacks exist with the type of thermoforming device described by Demerest. For example, a minimum amount of crystallinity, which is stated to be at least about 20%, must be obtained in the article to permit the article to be removed from the mold cavity without significant distortion. Thus, the device is not useful for applications where lower degrees of crystallinity may be desired in an article or a portion thereof. Moreover, the degrees of crystallinity actually obtained by using the Demerest apparatus typically are significantly higher than the stated minimum degree, and cannot be controlled effectively. Another drawback is that forming the sheet under tension results in distortion of the article after molding, which limits the ability of the apparatus to be used for many applications requiring especially high tolerances.
The device described in Demerest also is limited in terms of production speed. Following thermoforming, the articles undergo a series of cooling and drying steps prior to being separated from the mold cavities. The formed sheets are (again) tensioned to prevent distortion during separation from the mold. This type of procedure places severe limitations on production rates, especially for larger sized articles.
Gartland U.S. Pat. No. 4,469,270 describes a discontinuous thermoforming apparatus having a mold for thermoforming a plastic article having a flange portion. Vacuum and/or pressurized gas is used to conform a sheet to the shape of the heated mold. External cooling means are provided to maintain a portion of the flange of the article at a temperature which is said to be insufficient to induce undesirable thermal crystallization. This portion of the article preferably has a degree of crystallinity of not more than 10% to improve adhesion of lidding films to the article. The remaining portions of the flange and the remainder of the article preferably have the same average crystallinity.
It would be desirable to develop an apparatus for thermoforming articles which have excellent heat resistance and dimensional stability. It also would be desirable to develop an apparatus for thermoforming articles which exhibit excellent stress relaxation and which do not undergo distortion during cooling. It would be especially desirable to develop an apparatus capable of faster production times while substantially avoiding distortion, even for the production of larger sized articles.
The present invention is directed to an improved apparatus for thermoforming a thermoplastic article. The apparatus comprises a rotating wheel having one or more mold members. The mold member includes a molding surface, such as a male mold or a forming cavity (female mold) for receiving at least a portion of a substantially non-oriented thermoplastic sheet, and heat transfer means for heating and/or cooling the mold member to a temperature sufficient to maintain at least a portion of the thermoplastic sheet in a molten or thermoformable state. The apparatus also comprises a stripper plate which is maintained at a different (preferably lower) temperature than the mold member.
By maintaining the stripper plate at a controlled temperature, thermally-induced crystallinity in the adjacent portions of the sheet can be precisely controlled. This configuration permits the sheet to be formed without the need for tensioning or orienting, resulting in significantly improved product definition and, especially, retained product definition. It is particularly preferred that the sheet not be tensioned or oriented in either direction so as to eliminate post-mold distortion. The apparatus configuration also improves accuracy and precision of product trimming by reducing distortion normally attendant with changes in thermally-induced crystallinity as the sheet is heated and cooled during processing. As a result, the articles can be separated from the mold more rapidly following forming, which facilitates faster overall production rates.
The apparatus of the present invention additionally permits more precise control of thermally-induced crystallinity in thermoformed articles. The degree of crystallinity in products can be tailored to a particular application, e.g., in the manufacture of food containers such as microwave-ovenable containers, dual-ovenable containers, and the like. Significantly, the degree of crystallinity in the thermoformed article is not governed by manufacturing limitations. For example, unlike conventional continuous thermoforming devices, no xe2x80x9cminimumxe2x80x9d degree of crystallinity is required to enable the articles to be separated from the mold without distortion. Rather, the degree of crystallinity in an article can be selectively controlled in accordance with a degree most suitable for a particular application.
The apparatus also preferably includes one or more rolls or pairs of rolls for shaping and cooling the thermoplastic sheet from the extruder. The apparatus preferably has means for removing the entirety of the web, including the molded articles, from the forming cavity. In an alternative embodiment, the apparatus has means for separating (trimming) the molded articles from the plastic web while the articles are still in the mold, and means for ejecting each article individually from the mold.
In accordance with one preferred embodiment of the present invention, the mold member includes means for maintaining a predetermined temperature distribution therein. The predetermined temperature distribution results in thermoformed articles having at least two, and preferably at least three distinct regions of thermally-induced crystallinity. This can be accomplished, for example, by providing means for selectively heating portions of the forming cavity and, optionally, means for selectively cooling portions of the forming cavity. In addition, insulating elements can be provided to separate various components of the mold member to assist in establishing the desired temperature distribution.
While the plastic sheet in a thermoformable state is in contact with the mold, regions of the mold can be selectively heated to increase the rate of thermal crystallization or cooled to decrease the rate of thermal crystallization relative to other regions, to achieve the desired degree of crystallinity in each region. Within each region of the thermoformed article, either a substantially uniform degree of crystallinity can be present or, alternatively, a predetermined crystallinity gradient can exist. It has been found that thermoformed articles having excellent heat resistance and dimensional stability can be produced which also exhibit excellent stress relaxation, so as to essentially eliminate distortion during cooling without requiring excessively high degrees of crystallization. The apparatus of the present invention is particularly useful for making food containers, especially cooking containers such as ovenable food containers and dual-ovenable food containers.